A rapid and green approach to chiral α-hydroxy esters: asymmetric transfer hydrogenation (ATH) of α-keto esters in water by use of surfactants

Article abstract of DOI:10.1016/j.tetasy.2009.08.019

A series of α-hydroxy esters were rapidly prepared (1.5 h) from α-keto esters via asymmetric transfer hydrogenation (ATH) in water by the use of surfactants for the first time. This green method, catalyzed by a water-soluble and recyclable Ru(II) complex, gave moderate to high enantioselectivities (up to 99.7% ee) with DTAB as an additive and HCOONa as the hydrogen source.

Full text of DOI:10.1016/j.tetasy.2009.08.019

Tetrahedron: Asymmetry 20 (2009) 2033–2037
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Tetrahedron: Asymmetry
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A rapid and green approach to chiral
a-hydroxy esters: asymmetric
transfer hydrogenation (ATH) of
a-keto esters in water by use of surfactants
a,b
Lu Yin a,b, Xian Jia b, Xingshu Li a, , Albert S. C. Chan
*
a School of Pharmaceutical Science, Sun Yat-Sen University, Guangzhou 510006, China
b School of Pharmaceutical Engineering, Shenyang Pharmaceutical University, Shenyang 110016, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2009
Accepted 13 August 2009
Available online 16 September 2009
A series of
a-hydroxy esters were rapidly prepared (1.5 h) from a-keto esters via asymmetric transfer
hydrogenation (ATH) in water by the use of surfactants for the first time. This green method, catalyzed
by a water-soluble and recyclable Ru(II) complex, gave moderate to high enantioselectivities (up to
99.7% ee) with DTAB as an additive and HCOONa as the hydrogen source.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
esters and a,a,a-trifluoromethyl ketones using HCO2H/Et3N azeo-
trope as the hydrogen donor. Deng et al.5c introduced tunable den-
dritic N-mono-sulfonyl ligands to the ATH of ketones, olefins, and a
few examples of keto esters.
Asymmetric transfer hydrogenation (ATH) of ketones plays an
important role in asymmetric synthesis as it affords chiral
secondary alcohols which are valuable intermediates in organic
and pharmaceutical syntheses. Great efforts have been made to
achieve this reaction, especially transfer hydrogenation of prochi-
ral ketones1 in aqueous media, which has been the focus of
research over the past few years as a result of increasing require-
ments for environmentally friendly methods. Because of the insol-
ubility of organic substrates in water, phase-transfer catalysts or
surfactants were usually added to the reaction mixture. For
example, Chung et al.2a found that introducing surfactants to the
water-soluble Ru(II)-catalyzed ATH of ketones led to an increase
in the catalytic activity and enantioselectivity. Deng et al.2b
performed ATH of ketones in aqueous media containing micelles
and vesicles formed by the surfactant cetyltrimethylammonium
In contrast, to the best of our knowledge, no work involving the
rapid preparation of chiral
a-hydroxy esters by ATH in water has
been reported up to now. From practical and green chemistry
standpoints, it is significant to study the enantioselective synthesis
of chiral a-hydroxy esters by ATH of a-keto esters in aqueous med-
ia. We have recently disclosed a Ru-catalyzed enantioselective
preparation of methyl (R)-o-chloromandelate and its application
in the synthesis of (S)-clopidogrel (Fig. 1) by using Ru-(R,R)-2,4,6-
triisopropyl C6H2SO2-DPEN as the catalyst and HCOOH–Et3N azeo-
trope as the hydrogen donor.6 Herein, we report the first ATH of a
series of
a-keto esters catalyzed by Ru(II) complex in aqueous
media in the presence of surfactants.
bromide (CTAB) and obtained
a significant enhancement of
2. Results and discussion
activity, chemoselectivity, and enantioselectivity with the hydro-
phobic transition metal-amido complexes (TsDPEN-M) as the
catalyst.
In our previous work,6 we have attempted the enantioselective
preparation of methyl (R)-o-chloromandelate with PEG 400 or PEG
2000 as an additive to overcome the insolubility of the substrate
in water. However, analytical experiments showed that the ee val-
ues decreased compared with that of HCO2H/Et3N system. In-
spired by Chung2a and Deng’s work,2b we attempted to perform
On the other hand, chiral a-hydroxy esters are important phar-
maceutical intermediates and useful building blocks in asymmetric
synthesis.3 Although tremendous endeavors have been devoted to
the preparation of these compounds including chemical and bio-
logical transformations,4 there are very few examples concerning
ATH. Lemaire et al.5a afforded two cases of
a-keto ester in their
ATH of a series of
a-keto esters in water by using surfactants.
We chose methyl benzoylformate 4a as the model substrate,
(S,S)-Ts-DPEN 1, (S,S)-o-NO2–C6H4SO2-DPEN 2, and (S,S)-2,4,6-tri-
isopropyl C6H2SO2-DPEN 3 as chiral ligands (Fig. 2) for the Ru-cat-
Rh-catalyzed asymmetric reduction of prochiral ketones with
(1S,2S)-N,N0-dimethyl-DPEN as the chiral ligand. Mohar et al.5b re-
ported Ru(II)-catalyzed transfer hydrogenation of several
a-keto
alyzed ATH reaction of a-keto esters. The screening test including
variant surfactants, metal complexes, different loads of surfac-
tants, and reaction temperatures was carried out and the results
are listed in Table 1.
* Corresponding author.
E-mail address: xsli219@yahoo.com (X. Li).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.08.019

2034
L. Yin et al. / Tetrahedron: Asymmetry 20 (2009) 2033–2037
O
O
O
O
N
O
O
Cl
Cl
Cl
O
S
O
HO
O
S
O2N
(S)-clopidogrel
4b'
methyl (R)-2-chloromandelate
Figure 1. Asymmetric synthesis of (S)-clopidogrel.
H2N
HN SO2
H2N
HN SO2
H2N
HN SO2
NO2
(S,S)-Ts-DPEN 1
(S,S)-o-NO2-C6H4SO2-DPEN 2
Figure 2. Chiral ligands used for ATH of
(S,S)-2,4,6-iPr3-C6H2SO2-DPEN 3
a
-keto esters in water.
Table 1
ATH of methyl benzoylformate 4a in watera (screening test for optimal conditions)
O
OH
L*
[Ru(p-cymene)Cl2]2
O
O
HCOONa/H2O/surfactant
O
O
4a
4a'
Entry
Metal complex
Ligand
Surfactant
T (h)
Conv.b (%)
eeb (%)
1
2
3
4
5
6
7
8
9
10
11
12c
13d
14e
15
16f
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(benzene)]2
[RuCl2(p-cymene)]2
1
1
2
3
3
3
3
3
3
3
3
3
3
3
3
3
—
CTAB
CTAB
CTAB
SDS
DTAB
Triton X-100
STAB
DTAB/triton X-100 = 2:1
STAB/Triton X-100 = 2:1
SDS/DTAB = 2:1
DTAB
DTAB
DTAB
DTAB
DTAB
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
13.5
1.0
22
48
100
53
100
51
100
100
100
100
100
100
51
100
100
100
98
57.7
55.3
29.6
67.0
67.7
90.9
81.6
81.0
90.7
77.7
84.4
83.7
85.6
85.4
72.5
85.1
1.5
1.5
20
*
17
18
[RhCl2Cp ]2
3
3
DTAB
DTAB
1.5
1.5
100
100
86.1
48.9
*
[IrCl2Cp ]2
a
Reactions were carried out in 1 mmol scale at 28 °C with 50 mol % surfactants and S/C = 100 unless otherwise noted.
The conversions were estimated by TLC or 1H NMR analysis of the crude products and the ees were determined by HPLC on chiral OD-H column (hex: IPA = 90:10, 0.5 mL/
b
min).
c
Reaction temperature was 40 °C.
d
e
f
Reaction was performed at 0 °C.
With 10 mol % DTAB.
S/C = 500.
The catalytic activity increased when cetyltrimethylammonium
Based on the above elementary results, variant surfactants were
subsequently screened (Table 1, entries 5–11) with 3-[RuCl2(p-
cymene)]2 as the catalyst. The addition of sodium dodecyl sulfate
(SDS, an anionic surfactant) slightly enhanced enantioselectivity
(from 67.0% ee to 67.7% ee, Table 1, entry 5) but caused a decrease
in catalytic activity. Poly (ethylene glycol) mono 4-(1,1,3,3-tetram-
ethylbutylphenyl) ether (Triton X-100, a nonionic surfactant) and
steartrimonium bromide (STAB, a cationic surfactant) further
raised the ee values (81.6% ee for Triton X-100 and 81.0% ee for
bromide (CTAB) was introduced to the catalytic system 1-[RuCl2(p-
cymene)]2 (Table 1, entries 1 and 2). Catalytic system 2-[RuCl2(p-
cymene)]2, with a nitro group on the sulfonyl part, presented both
lower catalytic activity and enantioselectivity even in the presence
of CTAB (Table 1, entry 3). On the other hand, catalytic system 3-
[RuCl2(p-cymene)]2, which has three isopropyls on the sulfonyl
part, gave full conversion and 67.0% ee for the modal substrate (Ta-
ble 1, entry 4).

L. Yin et al. / Tetrahedron: Asymmetry 20 (2009) 2033–2037
2035
STAB, Table 1, entries 7 and 8), while another cationic surfactant
dodecyl trimethyl ammonium bromide (DTAB) gave the best result
(90.9% ee, Table 1, entry 6) comparable to that of the HCOOH/Et3N
system. A further study on the influence of different types of sur-
factants was performed (DTAB and Triton X-100 (2:1, molar ratio),
STAB and Triton X-100 (2:1, molar ratio), and SDS and DTAB (2:1,
molar ratio), but no greater effect on improving conversions as well
as enantioselectivities was observed (Table 1, entries 9–11).
A temperature effect was also investigated and results showed
that lower or higher reaction temperature was not favorable for
the enantioselectivity. When the reaction was carried out at 0 °C,
85.6% ee with full conversion was obtained with a little prolonged
reaction time (Table 1, entry 13). When the reaction was carried
out at 40 °C, the ee value decreased to 83.7% and no enhancement
in catalytic activity was observed (51% conversion for 1 h, Table 1,
entry 12).
It was found that lowering the load of surfactants was unfavor-
able for high enantioselectivity. When the amount of DTAB was re-
duced from 50 mol % to 10 mol %, 85.4% ee was afforded (Table 1,
entry 14).
Last but not the least, the metal precursor also influenced the
enantioselectivity of the reaction. When [RuCl2(benzene)]2 was
employed instead of [RuCl2(p-cymene)]2, only 72.5% ee was
obtained (Table 1, entry 15). A higher ratio of substrate to catalyst
(S/C = 500, with 3-[RuCl2(p-cymene)]2 as the catalyst) was also
tested (Table 1, entry 16), 98% conversion with 85.1% ee was ob-
tained when the reaction was quenched after 20 h. In addition,
Rh and Ir complexes were also used for the reaction, but did not
give better enantioselectivity, as expected (Table 1, entries 17
*
*
and 18, 86.1% ee for [RhCl2Cp ]2 and 48.9% ee for [IrCl2Cp ]2).
The results of the hydrogen-transfer reaction of a series of
a-
keto esters catalyzed by (S,S)-2,4,6-triisopropyl C6H2SO2-DPEN 3
under the optimal conditions are listed in Table 2. It appeared that
electron-withdrawing substituted aryl a-keto esters were unfavor-
able for high enantioselectivities. For example, methyl ortho- and
para-chloro benzoylformate gave lower ee values (Table 2, entries
2 and 3, 75.5% and 69.9% ee, respectively). In contrast, an electron-
donating group, methoxy at the para-position 4d seemed favorable
for the enantioselectivity (Table 2, entry 4, 90.1% ee). However,
ortho-substitution by the same group 4h dramatically reduced
enantioselectivity (33.0% ee, Table 2, entry 8), which may indicate
that steric hindrance played an important role in the enanti-
oselectivity of the reaction. This hypothesis was further proved
by the ATH of ethyl 2,4,6-trimethylphenylglyoxalate 4e, which
has two methyls at ortho-positions of the substrate, only 34.5%
ee was provided with 50% conversion over 20 h (Table 2, entry
5). The highest enantioselectivity was obtained by 4-methyl-
benzoic acid methyl ester 4g, which has an electron-donating
group at the para-position of the phenyl group (99.2% ee, Table 2,
entry 7).
Table 2
Asymmetric transfer hydrogenation of
a
-keto esters under optimal conditionsa
O
O
O
O
Cl
O
O
O
O
O
O
O
O
O
O
O
O
O
Cl
4d
4c
4a
4f
4e
4b
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4j
4g
4i
4h
O
O
O
O
Cl
O
Cl
O
O
O
O
O
O
O
4n
4m
4l
4k
Entry
Substrate
Product
T (h)
Conversionb (%)
eeb (%)
1
2c,d
3
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4a0
4b0
4c0
4d0
4e0
4f0
1.5
1.5
1.5
2
100
100
100
100
50
100
100
100
100
100
100
100
100
100
90.9
75.5
69.9
90.1
4
5e
6d
7
20
34.5
1.5
1.5
2
1.5
1.5
1.5
1.5
1.5
1.5
55.5 (63.2f)
99.2
4g0
4h0
4i0
8
9
33.0
88.5
90.0
83.7
91.1
78.6
77.0
10
11
12
13c
14g
4j0
4k0
4l0
4m0
4n0
a
Reactions were carried out in 1 mmol scale at 28 °C with 50 mol % DTAB and S/C = 100.
The conversions and ees were determined by HPLC on chiral OD-H column unless otherwise noted. Configurations were assigned by comparison with the retention times
b
in published literature.
c
Chiral OJ-H column (hex: IPA = 92:8, 1.0 mL/min).
(R,R)-2,4,6-Triisopropyl C6H2SO2-DPEN as the chiral ligand.
Chiral AD-H column (hex: IPA = 90:10, 0.5 mL/min).
S/C = 40.
d
e
f
g
Chiral OD-H column (hex: IPA = 95:5, 1.0 mL/min).

2036
L. Yin et al. / Tetrahedron: Asymmetry 20 (2009) 2033–2037
O
O
HN
O
H
O
N
OH
HO
H
O
O
O
ramipril
O
N
NH
H3C
O
N
HN
O
HOOC
O
enalapril
OH
O
quinapril
O
4f'
NH2
O
O
O
OH
NH
N
N
O
N
H
O
OH
O
OH
O
lisinopril
benazepril
Figure 3. ACE inhibitors derived from chiral ethyl 2-hydroxy-phenylbutyrate.
On the other hand, the influence of the ester group on enanti-
oselectivity was also investigated. Substrates 4a, 4i, and 4j, with
increased steric bulk gave almost the same level of enantioselectiv-
ities (Table 2, entries 1, 9, and 10, 90.9% ee for 4a, 88.5% ee for 4i,
and 90.0% ee for 4j). While 4b, 4m, and 4n, the ortho-Cl substituted
keto esters bearing an increased bulkiness of ester substitutions,
presented similar potency in enantioselectivities (Table 2, entries
2, 13, and 14, 75.5% ee for 4b, 78.6% ee for 4m, and 77.0% ee for
4n). Furthermore, 4-methyl-benzoic acid ethyl ester 4l, which
has merely a slight change at the ester part of 4g, afforded only
91.1% ee (Table 2, entry 12).
Table 3
The recycling test of catalysta
OH
O
L3*
[Ru(p-cymene)Cl2]2
HCOONa/H2O/DTAB
O
O
O
O
Run
1
2
3
4
5
T (h)
2.5
100
99.2
3.0
>95
98.6
6
10.0
50
98.6
24
50
89.9
Conv.b (%)
eeb (%)
85
99.7
Ethyl 2-hydroxy-4-phenylbutanoate 4f0 (Table 2, entry 6), an
important ACE inhibitor intermediate (Fig. 3), was also prepared
by this method. It is interesting to note that the reaction processed
smoothly in only 1.5 h with full conversion, much more rapidly
than that of HCOOH/Et3N system which required 24 h for the same
conversion.5b
a
Reaction conditions were the same as those used in Table 2, and 1.1 equiv of
HCOOH was added to regenerate HCOONa after each run.
b
The conversions and ees were determined by HPLC on chiral OD-H column
(Hex: IPA = 90:10, 0.5 mL/min, t1 = 17.85 min, t2 = 19.34 min).
Finally, the recycling of catalyst Ru-2,4,6-triisopropyl C6H2SO2-
DPEN 3 was tested with 4-methyl-benzoic acid methyl ester 4g as
the substrate and HCOONa as the hydrogen donor in water (Table
3). After each catalytic cycle, hexane was added to extract the
product and the residue containing the catalyst was reused by add-
ing pure formic acid to regenerate sodium formate for the next run.
As shown in Table 3, enantioselectivities remained high until the
4th run; however, conversion decreased to 50% with prolonged
reaction time (10 h). At the end of the 5th run, 50% conversion
was provided with only 89.9% ee over 24 h, which demonstrated
a loss in activity of the catalyst. The loss of catalyst during each
extraction process may be partly attributed to the hydrophobic
nature of the three isopropyls on the sulfonyl.
3. Conclusion
In conclusion, we have developed a practical method for rapid
preparation of a series of a-hydroxy esters in water by use of Ru(II)
complex-catalyzed asymmetric transfer hydrogenation and surfac-
tants as additives. The reactions in water were much faster than
that in HCOOH/Et3N azeotrope in organic solvent5b and high
enantioselectivities (up to 99.7% ee) were obtained. In addition,
the catalyst Ru-2,4,6-triisopropyl C6H2SO2-DPEN 3) could be re-
used until the 4th run without any loss in enantioselectivity. Fur-
ther study on the improved enantioselective preparation of the
important ACE inhibitor intermediate—ethyl 2-hydroxy-4-phenyl-
butanoate 4f0 is in progress.

L. Yin et al. / Tetrahedron: Asymmetry 20 (2009) 2033–2037
2037
filtered, and concentrated in vacuum to afford the crude product
which was purified by silica gel chromatography for determining
the enantioselective excess.
4. Experimental
4.1. General
The analytical data of ATH products of keto esters 4a,9 4b,6
4c,10 4d,11 4e,12 4f,13 4g,14 4h,12 4i,15 4j,16 4k,9 4l,10 and 4m17
were in agreement with those reported in the literature. ATH
product of 4n, (S)-isopropyl 2-(2-chlorophenyl)-2-hydroxyacetate.
The 1H NMR spectra were recorded with TMS as an internal
standard on a Bruker AV400 spectrometer. Ee values were deter-
mined on a SHIMADZU LC-20AT chromatography using Daicel Chi-
ralcel columns (OD-H, AD-H and OJ-H). The optical rotation was
measured with a Perkin–Elmer Model 341 polarimeter at 589 nm
and 20 °C, using a 1 dm path length. Pure water was used in ATH
½
a 2D0
ꢂ
¼ þ56:7 (c 0.8, CHCl3). 1H NMR (400 MHz, CDCl3): d 7.39–7.35
(m, 2H), 7.29–7.22 (m, 2H), 5.51 (s, 1H), 5.12–5.03 (h, J = 6.4 Hz,
1H), 3.74 (br s, 1H), 1.26–1.25 (d, J = 6.4 Hz, 3H), 1.11–1.10 (d,
J = 6.4 Hz, 3H); 13C NMR (CDCl3, 100 MHz): d 172.3, 136.2, 133.1,
129.4, 129.2, 128.3, 126.7, 70.1, 69.8, 21.2, 21.0. The enantiomeric
excess was determined by chiral HPLC, using a chiral OD-H col-
umn, hex: IPA = 95:5, 1.0 mL/min, 254 nm; t1 (major) = 6.88 min,
t2 (minor) = 7.99 min.
of
a-keto esters. Chiral ligands, substrates 4b, 4m, and 4n were
prepared as the procedure described in our previous work.6 Com-
pounds 4a and 4c–4f were purchased from Lancaster Chemicals
and used without further purification.
4.2. Preparation of substrates
Acknowledgments
4.2.1. Preparation of 4g, 4h, and 4l (method A)6,7 (modified
procedure)
We greatly appreciate financial support from the Science and
Technology Foundation of Guangzhou (07A8206031) and National
Science Foundation of China (20472116).
To a solution of p-tolualdehyde (8.04 g, 67 mmol) and tetrabu-
tyl ammonium bromide (TBAB) (1.6 g, 5 mmol) in chloroform
(12 mL) at 55 °C was dropwise added 50% NaOH solution (13 g of
NaOH dissolved in 13 mL of water) over 1.5–2 h. After stirring at
the same temperature for another 2 h, the reaction mixture was
cooled and diluted with water. The organic phase was separated,
acidified to pH 1 with 3 M HCl, and then extracted with ethyl ace-
tate. The combined organic phase was washed with water, dried
over anhydrous Na2SO4, and concentrated in vacuo to afford crude
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4.3. Typical procedure for ATH of
of surfactants
a-keto esters in water by use
A mixture of [Ru(p-cymene)Cl2]2 (3.06 mg, 0.005 mmol), (S,S)-
2,4,6-triisopropyl C6H2SO2-DPEN (5.74 mg, 0.012 mmol), and DTAB
(0.1542 g, 0.5 mmol) in degassed water (0.5 mL) was stirred and
heated at 40 °C for 1 h under argon atmosphere. After cooling to
room temperature, methyl benzoylformate 4a (0.1641 g,
1.0 mmol) and 2.5 M HCOONa (2 mL, 5.0 mmol) were subsequently
introduced. After the reaction was complete (monitored by TLC),
the reaction mixture was extracted with n-hexane for two times.
The combined extracts were dried over anhydrous sodium sulfate,
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